Preheat oven to 350 degrees. Lightly butter a 9-by-13-inch baking pan. Line with parchment paper, allowing a 2-inch overhang on the longer sides. Brush parchment with butter (not overhang).
Whisk together flour, baking powder, and salt in a medium bowl. In the bowl of an electric mixer fitted with the paddle attachment, mix butter and brown sugar until fluffy, about 2 minutes. Add eggs and vanilla; mix until well combined. Mix in flour mixture until combined. Fold in half of each of the marshmallows, chocolates, toffee chips, and caramels.
Spread batter in prepared pan. Scatter remaining marshmallows, chocolates, butterscotch chips, and caramels on top. Bake until top is golden brown and a cake tester inserted into the center comes out clean, about 35 minutes. Let cool on a wire rack. Lift out of pan, and transfer to a baking sheet. Refrigerate until set, at least 30 minutes.
Remove parchment, and cut into about 16 triangles. Bars can be stored in an airtight container at room temperature up to 1 week.
Substitute toffee chips with butterscotch chips.